Juve looked set for a 3rd straight away defeat once Ante Rebic’s volley place Milan ahead before the hosts lost Theo Hernandez to a second yellow card after seventy one minutes.
The guests offered very little threat despite their man advantage however the late VAR-awarded spot-kick allowed Ronaldo, thirty five last week, to make it thirty five goals in thirty five games for club and country.
The second leg in Turin is on four March.
Juve were knocked off top spot in Serie A at the weekend and their boss Maurizio Sarri would have been annoyed to see his side only manage 3 tame shots on target in a very tepid, scrappy game.
But within the last minute, Ronaldo saw his scissor kick blocked by the arm of Davide Calabria, who was looking the opposite way.
Referee Paolo Valeri didn’t initially award the penalty however pointed to the spot when going over to review the incident on the pitchside monitor. Ronaldo slotted his kick straight down the center.
Before the late drama it had been another veteran, 42-year-old Gianluigi Buffon, who had been the game’s star performer.
The Juve goalkeeper, who 1st won this trophy in 1999, made sensible saves from Rebic and Hernandez before an excellent one-handed stop pushed a Zlatan Ibrahimovic effort into the side-netting.
Shortly later Rebic struck to present Milan the advantage, turning in Samu Castillejo’s cross after a spell of pressure.
Hernandez picked up a second booking once he caught Paulo Dybala with nineteen minutes left and Ronaldo – who scored in a tenth straight league game at the weekend – then took advantage of the dubious decision to level in front of a new Coppa Italia semi-final record attendance of 72,738.
Napoli took a 1-0 lead over inter within the first semi-final, additionally played at the San Siro on Tues night.